The present invention relates to a photomask for use in forming a pattern in a photoresist layer; more particularly, to a photomask for obtaining a graded pattern profile on a photoresist.
Various techniques are known in the prior art for manufacturing a device on a semiconductor wafer, e.g., Si-substrate. Typically, photolithography processes are utilized in forming desired patterns on the wafer or a layer formed thereon.
In a photolithography process, a photoresist layer is applied first on an underneath layer to be processed and then the photoresist layer is exposed to light through a patterned photomask. By developing such processed photoresist layer, a pattern on the photomask is transferred onto the photoresist layer. The photoresist patterns generally have vertical side walls. When the photoresist patterns thus produced are used as etch masks in a subsequent dry etch step, the shapes of the holes in the photoresist patterns are transferred to the underneath etched layer, which, in turn, ceates holes on the underneath etched layer having vertical side walls.
Problems arise when depositing a thin film layer on such a structure. The deposited layer may be non-uniform or too thin, particularly in the region of edges or side walls of the holes, leading to a poor step coverage. Typical example of such phenomenon is a metal line with weak spots along side walls and edges of contact holes formed in an insulation layer. These defects are responsible for an increased failure rate, yield and reliability of the device.
One way to improve the step coverage of metal films is to make sloping side walls of holes in a patterned layer, e.g., by sputter-etching the side walls of the holes by using an inert gas, e.g., argon (Ar), after forming a barrier metal on the side walls of the holes.
FIG. 1 is a schematic cross sectional view showing a patterned oxide layer 2 having a hole 5, side walls of the hole 5 being inclined. The patterned oxide layer 2 is formed by: first, depositing an oxide layer on top of a wafer 1; patterning the oxide layer to form the hole 5 by using a photolithography technique; covering side walls of the hole 5 with a barrier metal layer 3; and sputter-etching the patterned oxide layer 2 to remove edges 4 of the hole 5 in order to obtain an inclined side wall profile.
There are certain deficiencies associated with the formation of the patterned oxide layer 2. First of all, the covering and sputter-etching steps should be added to the manufacturing process in order to solve the problem associated with the step coverage, which may add up the manufacturing cost and complexity. The thickness uniformity may also be compromised by the sputter-etching step.
Furthermore, since the side wall of the hole 5 is not of a rouneded profile but still has vertical portions at lower parts 7 thereof and angled edges between the upper and the lower parts 6 and 7, the inclined hole structure of the prior art shown in FIG. 1 may still be subject to low step coverage problems.
It is, therefore, an object of the present invention to provide a photomask capable of providing a smoothly rounded pattern profile on a photoresist layer.
In accordance with the present invention, there is provided a photomask for use in a photolithography technique, comprising: a transparent portion for transmitting light from a light source; an opaque portion coated with an opaque material for interrupting the light from the light source; and a border portion positioned between the transparent and the opaque portions, wherein the border portion includes a number of patterned structures coated with the opaque material and arranged along an edge of the opaque portion, the amount of light passing through the border portion gradually increasing when moving from a first borderline between the opaque and the border portions to a second borderline between the border and the transparent portions.